Long-term radar observations of the convective lines off the mountainous coast of southeastern Taiwan

The convective lines that frequently occur under relatively undisturbed weather conditions off the mountainous coast of southeastern Taiwan are a well-known but poorly understood mesoscale phenomenon. In this study, observations from the C-band Doppler radar on Green Island during a 7-year period from 1998 to 2004 were analyzed to show the general characteristics of the offshore convective lines in terms of their location, orientation and intensity. More than 200 cases of the lines were evidently identified in different seasons during the study period.

Few past studies of the offshore convective lines have shown that the low-level convergence produced as the offshore flow associated with the land breeze encountered the large-scale environmental onshore flow was a crucial forcing contributing to their formation. However, results from this study indicate that a significant number of the observed lines did not occurr close to the shore; instead, they tend to be located in offshore regions far away from the coast (>40 km). Moreover, their orientation exhibited a relatively wide range of angles and was not always parallel closely to the coast. These observational features cannot be reasonably explained by the simple interaction between the land breeze and the prevailing flow. It is thus suggested that the multiple, different mechanisms may exist and act to form the offshore convective lines. These important aspects, along with the possible role of coastal mountains in influencing the intensity and formation of the offshore lines, will be elaborated upon in the presentation.